I too am interested in putting an external SATA drive onto a late 2009 Mini. Per pricing, it would appear the 3TB-4TB are the best value, but what about 6TB? Where's the limit?
I guess, per the OP, these controllers obviously have some limit, but if as of 2006, 4TB was accessible (at least on the MacPro), where is the limit today on the nVidia? I've mostly been using this as a file server and I just bought a 13" Retina (2nd laptop since 2006 MBP 15" 1,1) so I'm not overly concerned about speed as much as consolidating storage. Would love to know what the easiest /most efficient way to add 4-8TB to one of these machines is. I see 2009 case top covers going for $15 shipped, so I don't mind drilling some holes to get the SATA cables out.
Seagate now sells a 2.5" 9mm 2TB drive for about $100, so perhaps just two of those and the OWC optical adapter is easiest, but it would be swell to have 8TB for nearly the same price.